Do I have to fit an appliance exactly to the manufacturers instructions?

Yes . All appliances must be fitted exactly to the manufacturers instructions. See Clause 6.5.
This means you cannot change a flue, or reduce the specified gas pipe size, and the inlet pipe must be correct as specified (i.e we can only use a hose if the appliance is “specified” to be used with a hose).
There are also other requirements within the standard: Once installed the appliance must be accessible for service (See Clause s 2.9.2 and 6.5.3) and the test point must be accessible for testing gas pressure (See Clause 5.1.7).
When installed and commissioned the test pressure must be correct. (See Appendix J)